Skeptically interrogating the universe with a fine understanding of human fallibility.

Friday, March 18, 2011

The MESSENGER Mission

Last night, as much of America was no doubt tuned in to the drama of NCAA basketball, I was pleased to be engaged otherwise. I just couldn't miss the big event. At 8 o'clock eastern time, NASA television carried live coverage of the MESSENGER orbit insertion maneuver (OIM), which was to be the first time any spacecraft has ever orbited the planet Mercury. It was a critical moment for the mission -- perhaps the most critical of its 6 1/2 years in flight -- and there was only one chance to get it right. Had the OIM burn failed, the spacecraft would have whizzed by Mercury, perhaps never to return. Fortunately, the maneuver seems to have been a total success.

NASA television is pretty cool, really. Their budget is clearly not incredibly high, but there are no commercials at all, and you get to enjoy an unfiltered look at our space operations. There was, of course, no live television images broadcast from MESSENGER, so the coverage consisted of interviews, animations, slide shows, and a live feed from mission control. It was all I needed to be content for the evening.

The last time Mercury was visited by any spacecraft was in 1975, when the Mariner 10 spacecraft made the last of its 3 flybys. Mercury is too close to the Sun to be observed by the Hubble telescope, so if we want to get a good look at it, we have to send a spacecraft. But because it orbits in such a hostile region of space, any spacecraft bound for Mercury must be engineered to deal with extreme temperature fluctuations; there is an almost 1100 degree Fahrenheit difference between light and shadow. Mariner 10 provided us with a lot of great science... among other things, it discovered that Mercury has a magnetosphere, totally unexpected amongst astronomers. But due to the timing of its flybys, and the nature of Mercury's slow axial rotation, it was only able to photograph about 45% percent of the surface of Mercury. The rest would remain a mystery. There would have to be another mission -- an orbital mission -- to map the rest of the planet, and answer some of the questions that were raised by the tantalizing results of Mariner 10.

For many years, another Mercury mission was not reasonable. It just cost too much, and there were lots of other places we wanted to go. Part of the problem with going to Mercury is that, contrary to what you might think, it takes a lot of delta-v to get there. Unlike missions to the outer planets, which have to muster up enough speed to climb away from the Sun, a trip to Venus or Mercury is falling toward the Sun, so you might expect that this requires relatively little propulsion capability. But remember, gravity is an accelerating force, so if you fly straight toward the Sun, you're going to pick up a lot of speed, and you would need an obscene amount of propellant to slow the spacecraft down to go into orbit. Mariner 10 was able to rendezvous with Mercury using a gravitational assist at Venus to slow it down and alter its trajectory to intercept Mercury (in fact, Mariner 10 was the first interplanetary spacecraft to use gravitational assists). But even then, Mariner 10 was incapable of orbiting Mercury.

MESSENGER's orbit maneuver was made possible by a complex series of gravity assists. The distance between the orbits of Mercury and Earth is only about 57 million miles on average. But as of yesterday, MESSENGER had traveled a total of 4.9 billion miles! That's almost 53 astronomical units... much farther than the distance to the orbit of Pluto. This is because MESSENGER spent 6 1/2 years traveling to Mercury, making almost 15 laps around the Sun: 1 flyby of Earth, 2 flybys of Venus, and 3 flybys of Mercury before finally approaching at such a speed that it could rendezvous and insert itself into orbit around Mercury. And even then, the insertion maneuver consumed 31% of the propellant carried at liftoff, leaving just 9% in the tank.

MESSENGER is a Discovery-class mission, which means it was made as cheaply as possible. But the gravity assists were just one way of saving money. To save additional propellant, the spacecraft also used its solar panels to make fine adjustments to its attitude and trajectory by using the radiation pressure of the Sun. Just like a solar sail, MESSENGER's solar array pivoted this way and that to use the winds of space to conserve energy. The spacecraft also needed a sunshade to keep its sensitive equipment cool, so the engineers gave it a heat shield made of ceramic cloth!

Sometimes, I think, we forget about Mercury. It is the innermost planet, and it is a scorching place, but it has no moons and no complicated atmosphere. It looks a lot like our own Moon, so maybe we feel like we've already sort of gotten all the information that we need. It certainly doesn't have the violent, morbid appeal of Venus, nor does it seem nearly as inviting as Mars, and compared with the giants of the outer Solar System, it may be a bit of an afterthought. But in spite of its relative proximity to Earth, we know so little about Mercury! Mariner 10 was just an appetizer... it began to show us what we don't know. Now it's time to find some of those answers. The 3 MESSENGER flybys have already given us breathtaking new images of the planet, far more detailed, and far more lovely, than anything sent back by Mariner 10. And there is much more to come.

We can hardly expect that the general public is going to get revved up about every peculiarity of Mercury. Mariner 10's results were a surprise, so now NASA hopes to answer key questions about the nature of Mercury's enormous, iron-rich core, its surface features, its magnetic field, and volatiles at the poles which may turn out to be water ice. But that may not be something you're going to go nuts about. What's important about planetary science is not always the little details that are revealed, but what those little details tell us about the bigger picture. When we are surprised in science, it means whatever model we have constructed is somehow incomplete or imperfect, so we go back and revise it. Sometimes they are little changes, but sometimes there are major implications for the field. In our search for exoplanets, for instance, we have learned that the structure of our Solar System -- terrestrials close in, gas giants farther out, with orbits more or less circular -- can hardly be considered typical, or the way it has to be. Rather, we have discovered gas giants with highly elliptical orbits, and enormous planets orbiting extremely close to their parent stars. So we have to revise our notion of how star systems may form. Similarly, the results from Mariner 10 and now MESSENGER reveal some gaps in our understanding of Mercury's formation, so we have to re-examine our assumptions.

Does the public care about that? Well, I think so. In spite of America's alleged apathy towards space science, there remains great enthusiasm for the latest discoveries. Though it seems clear that a large portion of the population is often in the dark about many of the latest astonishing breakthroughs, news of the most exciting discoveries tends to reverberate in all corners. It's rare that I meet someone wholly disinterested in the mechanics of the universe. In this case, the more we can learn about the formation of our solar system, the more we might come to understand how other star systems form, and that could perhaps give us some insight into where other life might exist, and what that life might be like. That alone is an awesome prospect, and it has profound implications for our understanding of who we are as a species, and as a planet. But we might also gain new insight into the nature of our Sun's formation, or the formation of the solar nebula, and that, in turn, could perhaps give us some clues as to what was going on in our region of the galaxy 4.5 billion years ago. As we connect these dots back in time, we are tracing our origins back, incrementally, to the beginning of the Universe. The MESSENGER mission will of course not be able to answer all of these questions, but it could help to answer some. And for 0.02% of your tax dollar, that's not bad.